Physical shipment of materials, including the shipment of mail, merchandise, raw materials, and other goods, is an integral part of any economy. Typically, the materials are shipped in a type of shipping containment or cargo box. Such containments or boxes include semi-trailers, large trucks, and rail cars as well as inter-modal containers that can be carried on container ships or cargo planes. However, such shipping or cargo containers can be used for illegal transportation of contraband such as nuclear and radioactive materials. Detection of these threats require a rapid, safe and accurate inspection system for determining the presence of hidden nuclear materials, especially at state and national borders, along with transit points such as airports and shipping ports.
Currently, both passive and active detection techniques are employed for the detection of concealed nuclear materials. Passive detection techniques are based on the principle that nuclear and radiological threats emit gamma, and in some cases neutron, radiation that can be detected. Although passive detection systems can be easily deployed, they suffer from a number of drawbacks, including high rates of false positives and misdetections caused by unavoidable factors such as depression of the natural background by the vehicle being scanned and its contents, variation in natural background spectrum due to benign cargo such as clay tiles, fertilizers, etc., and the presence of radio therapeutic isotopes in the cargo with gamma lines at or near threat lines. Further, many gamma sources are self-shielded and/or can readily be externally shielded, which makes them difficult to detect, since the radiation is absorbed in the shielding. Also, in general, gamma detectors make poor neutron detectors and good neutron detectors tend to be poor gamma detectors.
Other detection techniques employ uncharged particles, such as neutrons and photons (gamma rays) to irradiate suspicious containers. Uncharged particles have the potential to penetrate relatively large dense objects to identify particular elements of interest; thus, some detection devices utilize the absorption and/or scattering patterns of neutrons or photons as they interact with certain elements present in the object being inspected. Examples of such devices can be found in U.S. Pat. Nos. 5,006,299 and 5,114,662, which utilize thermal neutron analysis (TNA) techniques for scanning luggage for explosives, and in U.S. Pat. No. 5,076,993 which describes a contraband detection system based on pulsed fast neutron analysis (PFNA). All the aforementioned patents are incorporated herein by reference.
Active detection techniques, such as Differential Dieaway Analysis (DDA) and measurements of delayed gamma-ray and neutrons following either neutron- or photon-induced fission, can be used to detect the presence of fissile materials. The radiation is measured with neutron and gamma-ray detectors, preferentially insensitive to each other's radiation. Detection of delayed neutrons is an unequivocal method to detect fissile materials even in the presence of shielding mechanism(s) to hide the nuclear materials and notwithstanding the low background compared to delayed gamma rays. Because the number of delayed neutrons is two orders of magnitude lower than the number of delayed gamma rays, efficient and large area detectors are required for best sensitivity in neutron detection.
Each of the detector systems described above is not without drawbacks. In particular, these devices generally utilize accelerators that produce high energy neutrons with a broad spectrum of energies. The absorption/scattering of neutrons traveling at specific energies is difficult to detect given the large number of neutrons that pass through the object without interaction. Thus, the “fingerprint” generated from the device is extremely small, difficult to analyze, and often leads to significant numbers of false positive or false negative test results.
In addition, known prior art detection systems have limitations in their design and method that prohibit them from achieving low radiation doses, which poses a risk to the personnel involved in inspection as well as to the environment, or prevent the generation of high image quality, which are prerequisites for commercial acceptance.
While the use of both passive and active detection techniques is desirable, what is needed is a neutron and gamma-ray based detection system and method that is cost-effective, compact, and wherein the neutron detector is fabricated from readily available materials.
The most commonly used neutron detector is a He-3 gas proportional chamber. Here, He-3 interacts with a neutron to produce a He-4 ion. This ion is accelerated in the electric field of the detector to the point that it becomes sufficiently energetic to cause ionisation of other gas atoms. If carefully controlled, an avalanche breakdown of the gas can be generated, which results in a measurable current pulse at the output of the detector. By pressurizing the gas, the probability of a passing thermal neutron interacting in the gas can be increased to a reasonable level. However, He-3 is a relative scarce material and it does not occur naturally. This makes the availability and future supply of such detectors somewhat uncertain. Further, a special permit is required to transport pressurized He-3 tubes, which can be cumbersome and potentially problematic.
The most common globally deployed passive radioactive material detectors employ a neutron moderator 105 in an upper portion, having a plurality of He-3 detector tubes 116 embedded therein covered by a lead shield 108 and a lower portion comprising a plastic scintillator and moderator 110 with a PMT (Photo Multiplier Tube) 115 embedded therein, as shown in FIG. 1A. This detector configuration, however, still employs the scarce He-3. In addition, another commonly deployed detector where the gamma-ray and neutron detectors are separate is shown in FIG. 1B. As shown in FIG. 1B, neutron moderator 105, comprising a plurality of He-3 detector tubes 116 is positioned adjacent to plastic scintillator 110, comprising a PMT 115 and a lead shield 108. This detector configuration, however, still employs the scarce He-3 and takes up a larger footprint.
Several alternative detectors to replace He-3 detectors have been identified. However, many of these detectors are also sensitive to gamma rays, which is not acceptable in applications where neutrons must be discriminated from gamma rays.
Therefore, what is needed is a neutron and gamma-ray based detection system and method that is cost-effective, compact, and wherein the neutron detector is fabricated from readily available materials. In addition, what is needed is a cost-effective and compact detection system in which neutron and gamma-ray detectors are separate.